Geographic patterns of diversification and the latitudinal gradient of richness of rocky intertidal gastropods: the 'into the tropical museum' hypothesis

Aim To evaluate the existence of a latitudinal gradient of richness (LGR) in rocky intertidal gastropods and the role of evolutionary processes in shaping the LGR. Location The entire eastern Pacific coast, from Alaska to Tierra del Fuego, encompassing ca. 120 degrees latitude. Methods The LGR was assessed using field surveys and a comprehensive literature dataset from 308 rocky intertidal sites, representing 328 species and 159 genera. The importance of geographic patterns of diversification in shaping the LGR was assessed using three complementary approaches. First, we used the fossil record to evaluate the region of origination of genera (tropical vs. extra-tropical). Secondly, we used a nestedness analysis to compare present-day biogeographic distributions of genera with their region of origin. Finally, we evaluated the importance of biogeographic dynamics for shaping the LGR using a projection matrix and estimating transition probabilities among different biogeographic states. Results Eastern Pacific rocky intertidal gastropods follow the canonical latitudinal gradient of richness (higher richness in the tropics). Originations were similar in the tropics and extra-tropics, a pattern that was robust to geographic differences in fossil sampling. The biogeographic distribution of genera was significantly nested, irrespective of the region of origin of genera. The distribution dynamics model accurately reproduced the LGR and showed that key-transitions are low extinction in the tropics and high dispersal of taxa from the extra-tropics to the tropics. Conclusions The existence of a canonical LGR in rocky intertidal gastropods can be explained by the combined effect of reduced extinction rates in the tropics and the range expansion of taxa from the extra-tropics toward the tropics, in what we have called the 'into the tropical museum' hypothesis.